While manufacturers still look toward stainless steel for many vacuum applications, the value and benefits of using aluminum, particularly for ultra-high and extreme vacuum applications, are important to note. Aluminum can often outperform the older technologies used for vacuum chambers on a number of levels (see Figure 1.) We’ll go over the most significant and consequential benefits to prove the superiority of aluminum for use in your next vacuum system application.
Thermal conductivity. Consider, for example, that aluminum exhibits excellent thermal properties, providing the user ten times the thermal conductivity as stainless and twenty-one times the thermal diffusivity of stainless. Aluminum also displays extremely low thermal emissivity rates. Excellent thermal properties such as these unmatched thermal performance characteristics make aluminum an excellent vacuum material. The surface properties of aluminum, which used for a vacuum chamber, allow full bakeout at 150oC, which is much lower than what is needed as the bakeout temperature for stainless steel. And since aluminum chambers heat up uniformly, bakeouts are performed more complete and with significantly reduced cycle times because there are fewer cooler zones to recondense gases.
Ultra-pure vacuum. Hydrogen is the predominant residual gas in metal vacuum systems in the ultra-high and extremely high vacuum. The reduction in the hydrogen outgassing rate is the most challenging problem in achieving extremely high vacuums when using stainless steel. With seven orders of magnitude less hydrogen outgassing than stainless steel, aluminum provides the user with an ultra-pure vacuum. The material has very low levels of carbon, resulting in significantly less H2O, CO, C2, and CH4 than stainless steel.
Reduced Contamination and Outgassing Rates
An aluminum chamber processed according to Atlas UHV specification AVSP-08 entails cleaning surfaces to facilitate the formation of a dense passivation layer throughout, which allows for the conversion of hydroxides into stable oxide molecules. This activity results in a surface that inhibits the diffusion of other contaminants, which further reduces pumping requirements. Faster pumping equates directly to smaller and less expensive pumps, ultimately saving costs in both expensive pump purchases as well as energy usage (see Figure 2.) A baked aluminum chamber has an outgassing rate of less than 1x10-13 Torr liter/sec cm2 compared to stainless at 6.3x10-11 Torr lier/sec cm2—making aluminum suitable for extreme high vacuum applications.
No magnetic interference. For applications that require their chambers to exhibit non-magnetic capabilities, aluminum offers an essentially magnetic transparent solution. An aluminum chamber built to Atlas UHV standards provides low magnetic permeability so that there is no measurable disruption to electron and ion optics.
Less residual radiation. Fluorine gas is a common cleaning agent in aluminum chambers. Due to the company’s AVSP-08 process—that forms a dense protective layer that make the aluminum a highly corrosive resistant material—their aluminum chambers and gas delivery lines are far more resistive to fluorine than those made of stainless steel. Surfaces can be further protected from halogens by producing an even thicker and harder oxide layer through an electrolytic anodizing process if required by the user.
High-speed machining. Aluminum is known to provide superb machinability characteristics allowing it to be machined up to ten times faster than stainless steel. Chambers made from aluminum can also be created with more detail (see Figure 3.) Aluminum can be cut, shaped or formed and extruded quite easily. Chamber features are produced to fit an application precisely rather than tailoring an application to fit a material’s manufacturability limitations, often reducing the need for additional space and equipment.
Vibration damping. Aluminum provides high vibration dampening and increased absorption. With a low Young’s modulus (69GPa) of elasticity (1/3rd that of stainless steel, 207GPa) aluminum offers outstanding vibration dampening characteristics to the user. This benefit is particularly important, and making aluminum the material of choice, when used for applications such as those in precision synchrotron, semiconductor, and physics applications where excess vibration can have disastrous consequences.
Material costs. Probably the greatest savings is the lower cost of the material, which greatly improves your bottom line. Compact aluminum vacuum chambers can offer up to a 40 percent smaller footprint to provide an economical alternative to bulky stainless-steel systems—especially valuable when floor space is at a premium. Add to this the fact that aluminum weighs a third of the weight of a stainless-steel chamber of the same size and you have lower shipping costs and easier product handling operations. Lighter weight means that aluminum vacuum chambers require less expensive support structures, while the lower weight helps in speeding installation times.
Integrating components and all-metal flange seals
When incorporating an aluminum chamber into your present design it is often necessary to provide a juncture between aluminum and stainless-steel that will adequately seal the two together. Metallurgically bonded (explosion or diffusion bonding) bimetallic transitions provide a hermetic, vacuum tight, seal that enables cryogen, liquid or gas supply lines to interconnect between one metal. These transition joints can be either entirely welded or flanged joints. Bimetal flanges provide a robust all stainless sealing face capable of crushing a copper gasket on an aluminum body for weld-up to an aluminum chamber. Bimetallic joints enable engineers to pick and choose which materials they need and where they need them to perform their particular function within the overall system (see below, “Explosive Bonding Process and Diffusion Bonding Process”.) These metal-to-metal transitions are known to withstand the pressures and temperatures that are used in handling liquid nitrogen, hydrogen, helium, and many other industrial gases and liquids for a wide variety of reasons. Adhesives are not used for these applications due to their high outgassing rates and, in cryogenics, due to embrittlement.
When considering the entire cost of a vacuum chamber, from shipping to installing to operation, an aluminum chamber can save users between 40 and 60 percent when compared with stainless-steel vacuum chambers. The larger the chamber, the more pronounced the savings. Further, when fitted with the company’s flanges and fittings treated with their AVSP-08 process, systems cost less than a comparably equipped stainless-steel system. Because the thermal, machining, and materials cost advantages are so clear the semiconductor industry uses aluminum for much of its wafer production tools. Because Atlas products are compatible with all standard stainless steel vacuum equipment, they have been able to integrate smoothly into a wide variety of markets.
Explosion Bonding Process and Diffusion Bonding Process
Explosion bonding or welding (EXW) is a solid-state process by which dissimilar metals can be joined together at an atomic level. Preparation for bonding requires that plates lay flat against each other—a flyer plate on top of the base plate separated by a small gap. An explosive charge is placed on the flyer plate and detonated from a point at the edge of the plate. A controlled progressive ignition travels across the flyer plate like ripples in a pond. The explosion accelerates the plates together with impact velocities of 1800-2200 m/sec. A high energy surface plasma is formed between the plates, moving ahead of the collision point and stripping electrons from the two bonding surfaces. The electron-hungry metals are then thrust together at extreme pressures forming an electron-sharing bond.
Metals like copper and stainless steel can be readily bonded through the EXW process. However, aluminum and stainless steel are incompatible and not directly bondable, because of the formation of brittle intermetallic compounds. Atlas UHV has researched and developed a patented multi-layer composites technology that solves this challenge (see Figure 4.) Their solution exploits the metallurgical compatibility from a multi-layered composition consisting of 316L stainless steel, copper, titanium, and 6061 T6 aluminum to provide a part with maximum hermeticity, ductility, and ability to cycle from cryogenic to high temperatures.
Diffusion bonding is a process by which different metals are placed together with very high pressure and heated to an elevated temperature for a specific duration. Bonding occurs in stages. First the materials yield and creep in a way that the frictional force pushes waves of plastically deforming material into a larger area of contact. At this time, atoms in the contact area diffuse and rearrange the boundaries of the two materials in such a way that it eliminates the pores originally in the bonded area. Finally, this diffusion dominates the area, and the bond is formed.
This method of bonding depends on the strict control of bonding pressure, bonding temperature, and the holding time. These conditions are different for bonding different materials. Aluminum stainless bonds are particularly challenging due to formation of brittle intermetallic compounds that weaken the bond. Working with a company that has experience with a variety of bonding methods and bonding materials will help assure that your needs are met. Once the bond is made, it takes unique expertise to properly machine the joined material into a useable part.
Jed Bothell is the Vice President of Atlas UHV, a manufacturer of proprietary ultra-high vacuum products. Contact Bothell at [email protected]